> [...] Is there a qualifier [...]

   No, only a file specification.

> x.y. selects the file x.y with the highest version number.

   "x.y;" or "x.y;0" would probably be clearer (and might be safer for
an ODS5 volume).

> $ sd [source]
> $ backup/compare tape:abc.bck [...]

   Checking the actual tape is probably admirable, but I'd probably
start by comparing the directory tree used to make the tape with the
development tree:

      backup /compare [source...]; [development...];

(Add "device:" spec's, as appropriate.)

> $ backup/ver [source...]*.* tape:abc.bck/sav

   I'd probably be using "[source...]*.*;" here, too.  Do you want
multiple versions of any files on the tape, or have you used PURGE on
this tree, or what?